A switched reluctance machine (SRM) is a brushless, synchronous machine having salient rotor and stator poles. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. Each pair of diametrically opposite stator pole windings is connected in series or in parallel to form an independent machine phase winding of the multiphase SRM. Ideally, the flux entering the rotor from one stator pole balances the flux leaving the rotor from the diametrically opposite stator pole, so that there is no mutual magnetic coupling among the phases.
Torque is produced by switching current in each phase winding in a predetermined sequence that is synchronized with angular position of the rotor. In this way, a magnetic force of attraction results between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative, or braking, torque. In a SRM, torque direction is independent of current direction. Therefore, in contrast to most other brushless machines which require bidirectional phase currents, a SRM power inverter can be configured to enable current flow in only one direction through a phase winding. Such an inverter generally employs one or more switching devices, such as transistors or thyristors, in series with each machine phase winding. Advantageously, the switching devices prevent "shoot-through" current paths. Exemplary SRM converters are illustrated in commonly assigned U.S. Pat. No. 4,684,867, issued to T. J. E. Miller on Aug. 4, 1987, which is hereby incorporated by reference.
At relatively low and medium speeds, current magnitude regulation in SRMs is typically achieved by hysteresis band current chopping. In a SRM drive employing two switching devices per phase, such a current chopping scheme involves generating a commanded reference current waveform which has predetermined upper and lower hysteresis band limits to which the phase currents are continuously compared. At the start of a conduction interval for one phase (i.e., when a phase is excited for torque production), the switching devices in series with the corresponding phase winding are simultaneously switched on. With both switches thus conductive, current from the DC source builds in the phase winding until the upper limit of the hysteresis band is reached. At that point, both switching devices are turned off. Flyback or return diodes coupled to the phase winding provide a current path back to the DC source. When the phase current decreases to the lower limit of the hysteresis band, the switching devices are switched on again, and the process repeats. This process is commonly referred to as pulse width modulation (PWM) or current chopping. Such a hysteresis band current chopping strategy is described in commonly assigned U.S. Pat. No. 4,739,240, issued to S. R. MacMinn and P. M. Szczesny on Apr. 19, 1988, which is hereby incorporated by reference.
Disadvantageously, high frequency current chopping results in a ripple current component on the DC source bus which must be removed by the DC bus filter capacitor. The size and weight of the filter capacitor are directly proportional to the ripple current rating thereof. Therefore, in order to reduce the volume and weight of a SRM drive, it is desirable to reduce the maximum allowable ripple current. Further, it is desirable to reduce switching losses and hence junction temperatures of the switching devices. In particular, since power is dissipated in a switching device each time the device transitions between a conductive and a nonconductive state, switching losses can be reduced by decreasing the chopping frequency.